This invention relates generally to techniques for providing a stable scale factor in a fiber optic rotation sensor having a light source that provides counter-propagating waves to a fiber optic sensing coil. This invention relates particularly to techniques for stabilizing the scale factor in a fiber optic rotation sensor by stabilizing both the polarization of the optical signal from the light source to the interferometer and the polarization of optical signals fed back from the interferometer to the light source.
A fiber optic rotation sensor includes an optical signal source that provides counter-propagating waves to an interferometer that includes a fiber optic sensing coil. A typical optical signal source includes a laser diode that provides pump light to a gain fiber. The gain fiber includes a dopant that absorbs pump light and then emits an optical signal that is suitable for delivery to the interferometer. Within the interferometer an integrated optics chip is connected to the fiber optic sensing coil. A typical integrated optics chip in a fiber optic rotation sensor includes components such as polarizers, phase modulators and a Y-coupler that are used in processing and controlling optical signals input to and from the fiber optic sensing coil. The output of the interferometer is the phase difference between the two counter-propagating waves. The rotation rate of the coil about its sensing axis is obtained by dividing this phase difference by a scale factor. The scale factor of a fiber optic gyroscope (or Sagnac scale factor, SSF) is given by the well-known equation: ##EQU1## where L is the length of the sensing loop (including the coil length plus the length of the integrated optic waveguides after the Y-coupler or Y-junction, D is the effective coil diameter, c is the speed of light in vacuum and .lambda. is the "average" or "interferometric" mean wavelength. Making accurate measurements with a fiber optic rotation sensor requires an accurate and stable scale factor.
The scale factor stability of fiber optic gyros is affected by changes in the polarization state of the light in the fiber between the optical source and the integrated optics chip. Changes in stress within the fiber will cause the polarization sate of light guided by the fiber to change. This stress may be mechanical or thermal in origin. Any change in polarization state changes the scale factor of the fiber optic rotation sensor via optical filtering of the light when the traversing first the section between the source and the integrated optics chip (made partly of the single mode fiber and partly of polarization maintaining fiber) then traversing the polarizing integrated optics chip itself. This optical filtering can lead to short term scale factor instability and long term degradation of the scale factor repeatability.